1. Field of the Invention
The present invention relates to a zoom lens device suitable for use in a silver salt camera, a digital camera, a video camera, or the like, and an optical apparatus provided with the zoom lens device.
2. Description of the Related Art
Smaller and lighter home video cameras or the like have caused imaging zoom lens devices to be greatly reduced in size. In recent years, energy has been particularly put into, for example, shortening the length of the entire zoom lens device, reducing the diameter of the front lens, and simplifying the structure of the optical system.
The so-called rear focusing type zoom lens device which performs focusing by moving a lens unit other than the first lens unit at an object side is known as a means for attaining these objects. In general, compared to the zoom lens device which performs focusing by moving the first lens unit, the rear focusing type zoom lens device can have its first lens unit formed into a smaller effective diameter, thus facilitating reduction of the overall size of the lens system.
Such rear focusing type zoom lens devices, disclosed, for example, in Japanese Patent Laid-Open Nos. 62-24213 and 62-247316, comprise in order of lens units from the object side, a positive first lens unit, a negative second lens unit, a positive third lens unit, and a positive fourth lens unit. The second lens unit is moved to change the magnification, and the fourth lens unit is used for correcting changes in the image plane occurring as the magnification changes and for focusing.
On the other hand, there is an increasing need for a zoom lens device that can achieve magnification changes of more than 10. In order to achieve high magnification changes, while minimizing aberrations, however, it is necessary to use a large number of lenses for each of the lens units to cause each of the lens units to play a smaller part in correcting aberrations. Using a larger number of lenses for each of the lens units obviously increases the overall size of the zoom lens device. Therefore, in this case the zoom lens device cannot be reduced in size and at the same time made to attain high magnification changes.
Size reduction and a high magnification change can both be achieved by a known conventional method in which a surface of a lens of the zoom lens device is formed into an aspherical surface. However, when the magnification change is high at more than 10, it becomes difficult to correct chromatic aberration of a lens with an aspherical surface.
Proper correction of chromatic aberration and size reduction of the zoom lens device can be achieved by another known method which uses a diffraction optical element. Such a method is disclosed, for example, in U.S. Pat. No. 5,268,790. In this method, a diffraction optical element is used in the second lens unit, being a variator, and/or the third lens unit being a compensator.
Accordingly, it is an object of the present invention to provide a zoom lens device reduced in size and constructed in a way unknown in the prior art to achieve a high magnification change and to allow chromatic aberration to be properly corrected.
To this end, in a preferred form of the present invention, there is provided a zoom lens device in which a lens unit which is stationary when the magnification is being changed includes a diffraction optical element.
In the zoom lens device of the preferred form of the present invention, when the lens unit which is stationary during magnification changes is the lens unit closest to the object side, the diffraction optical element has a diffraction optical surface at the image plane side.
In another preferred form of the present invention, there is provided a zoom lens device having an overall positive refractive power and comprising in order of lenses from a long conjugate side, at least a first lens unit, a second lens unit, and a third lens unit. During zooming, the size of the image is changed by moving the second lens unit, and changes in the image plane occurring as the magnification changes is corrected by moving at least one of the lens units among the third lens unit and onwards. The first lens unit includes a diffraction optical element.
In still another preferred form of the present invention, there is provided a zoom lens device comprising in order of lens units from a long conjugate side, at least a first lens unit, a second lens unit, a third lens unit which is stationary during zooming, and a fourth lens unit. The size of an image is changed by moving the second lens unit, and changes in an image plane occurring as the magnification changes is corrected by moving at least one of the lens units among the fourth lens unit and onwards. The second lens unit includes a diffraction optical element.
In still another preferred form of the present invention, there is provided a zoom lens device comprising in order of lens units from a long conjugate side, at least a first lens unit, a second lens unit, a third lens unit which is stationary during zooming, and a fourth lens unit. The size of an image is changed by moving the second lens unit, and changes in an image plane occurring as the magnification changes is corrected by moving at least one of the lens units among the fourth lens unit and onwards. The third lens unit includes a diffraction optical element.